The vast majority of the planet’s ice-free land surface – 83 per cent according to one study (pdf) – is now influenced by humans in some way or another. Where I live, in the British Isles, no part of the landscape is totally unaltered by people. The Lake District, for example, if left ungrazed by sheep, would revert to dense woodland on all but the highest peaks. Throughout the entire United Kingdom, the only species that have survived into the modern era are those that are able to coexist with human domination of the land: others, from beavers to wolves, have been extirpated entirely.

Human impacts on land may be much greater than is obvious at first sight. Roads, for example, appear to directly affect only a relatively small strip of land, but they also cut ecosystems in half, altering the survival prospects of species living on either side of them. With an estimated 1 million animals killed every day on America’s road network, the effect of this constant removal of predators and prey is felt over much wider areas. A seminal 2002 study of the ecological effects of a busy four-lane highway in Massachusetts found impacts – varying from wetland drainage to noise – across a broad 600-metre corridor. The consequent nationwide effects over the United States’ entire 6.2-million-kilometre road network can only be guessed at.

Whilst busy paved roads are a recent phenomenon, general human transformation of the land surface has been accelerating for millennia. The Roman empire deforested large areas around the Mediterranean, contributing to soil erosion and declining fertility. The European continent’s landscape changed dramatically between ad 500, when it was still four-fifths covered by swamp and woodland, and ad 1300, when half of this natural land had already been converted to agriculture. This transformative process fluctuated in lockstep with human population growth: when the Black Death killed a third of Europe’s population in the early fifteenth century, forests stopped their decline and began to regrow. To this day, many of Germany’s most valued ‘natural’ woodlands owe their existence to the depopulation wrought by the medieval plague.

With so little of the Earth’s land still pristine and unaffected by humans, the idea of the ‘wilderness’ has less and less meaning in the modern world. Indeed, if pollution and climate change are taken into account, no part of the planet’s surface is any longer truly wild. This does not mean that we must gloomily accept the continuing diminution of semi-wild areas and the erosion of the vital ecosystem services they provide. It does mean though that we need to challenge some orthodoxies that are no longer useful in this new era of near-total human planetary dominance. ‘Getting close to nature’ or going ‘back to the land’ will generally not be good for the environment, however psychologically fulfilling these objectives may be to individuals seeking escape from industrial living. Instead, we need to intensify agriculture and other human land uses in existing areas as much as possible, and encourage as an environmental boon the growth of the world’s major cities that already successfully concentrate today’s enormous human population onto only a tiny proportion of the world’s land. The most positive trend of all in allowing us to minimise our impact on the planet’s surface is one more often bemoaned than celebrated: urbanisation.

Land and freedom
The human transformation of land has been extremely good news for our species overall, allowing a population now approaching 7 billion people to be supported at ever-rising standards of living and comfort. Each of us now lives like a medieval king, offered choices of sumptuous foods from around the globe and surrounded by machines that wash our clothes and dishes at the touch of a button.

But, for land in particular, our impacts on the planet’s terrestrial surface are now so extensive and transformative that they threaten the capacity of varied ecosystems to self-regulate and maintain the living biosphere overall. Natural landscapes filter water, produce oxygen, sequester carbon, and provide vital habitat for other species. Whether these landscapes harbour forests, wetlands, grasslands or even deserts, the planet’s varied biomes are an integral part of the Earth system – and are essential to its continued sustainable functioning.

Computer models of the Earth system can demonstrate how varied ecological zones are important to the planet overall. In one modelling study, climatologist Peter Snyder and his colleagues tried deleting entire biomes from their simulated planet to see what then happened to the climate. Remove the savannahs, they found, and you reduce rainfall totals by a third. Press the delete key over the tropical forests, and entire continents see dramatic reductions in their precipitation and sudden rises in temperature. Different biomes, from temperate forests to the Arctic tundra, likely have significant effects on winds, temperatures and atmospheric circulation patterns.

As more and more of the Earth’s surface sees these traditional biological zones transformed into ‘anthropogenic biomes’ dominated by humans, Earth-system scientists expect the planet’s functioning to become steadily more unstable. Across at least three-quarters of the world today, humans are as or more important in determining which species make up the ecosystem than previously dominant factors like latitude, climate and geography. Instead of the neat green and yellow strips that we learned about in geography class, denoting jungle, desert and so on, the planet is now divided into a complex mosaic of different human-influenced ecosystems. Human capture of the planet’s productive capacity, according to the most recent relevant study, now adds up to about 24 per cent of the total productivity of the Earth’s terrestrial biosphere. In other words, a quarter of everything produced by all plants on land is eaten or otherwise consumed by us.

Different landscapes are exploited with different intensity: whilst forests will yield up to a fifth of their annual production in fuel, fibre or timber, cropland allows us to grab an impressive 83 per cent of the yearly productive share per hectare. The human species, therefore, whilst comprising only half of 1 per cent of the global animal biomass, consumes a significant fraction of everything the Earth produces. This triumph for us is of course a disaster for the species we have displaced from their food webs: the disappearance of habitat and food supply is currently the greatest cause of the planet’s continuing loss of biological diversity. It is also clear that ecological tipping points can be crossed if we push this process too far, with potentially irreversible consequences as overgrazed grassland tips into desert, or as degraded tropical forest dries out and burns over vast areas of Indonesia and Brazil.

How far this trend can continue before precipitating some kind of regional or even global collapse in the functioning of the biosphere was considered by the planetary boundaries expert group, which concluded that no more than 15 per cent of the Earth’s surface should be converted to cropland in order to protect the Earth system as a whole. We are perilously close to this proposed planetary boundary already, with 12 per cent of land already devoted exclusively to agriculture. This leaves only 400 million hectares of additional land to be brought into production, a major challenge for humanity given population growth and increases in wealth and consumption across the planet. It is a challenge we can still meet, however – but only if we make the right decisions about how to use the planet’s land most wisely.

Meat and energy
There are two major current trends that mean the world must continue to produce more and more food from a limited area of land, probably necessitating a doubling of production within forty years. The first is the growing world population, which will reach 9 billion or more people by mid-century. The second is the tendency of more prosperous populations to consume more food in general, and to increase the proportion of meat and dairy in their diets. It is important to recognise that there is almost nothing we can or should do to influence either of these trends inasmuch as they affect the environment.

People’s desire to eat more meat as they get more wealthy is so deeply embedded in most cultures (and getting lots of protein may even be a biological impulse inherent in all of us) that it is not something that is amenable to outside influence. As with climate change, the only pragmatic option is to concentrate efforts to fulfil people’s desires and demands in a way that protects natural ecosystems as far as possible – not to try to challenge patterns of consumption per se by insisting that they are unsustainable, even if this appears to be the case in the short term. Such an approach has failed in the past and will continue to fail in the future.

Hand in hand with the protection of high-biodiversity and wilderness areas must come a better use of the land that is devoted to human-centred agriculture. Because organic agriculture produces on average only half the yield of crops per unit of land as conventional farming, any mass conversion to organic would end up using much more land. Instead, intensification – producing more yield per unit of existing farmland – using advanced farming technologies and high-yielding varieties of crops offers a more promising route to feeding a larger and more prosperous human population. Even so, some additional land will need to be brought into production to satisfy future increases in demand. The planetary boundaries expert group suggests a focus on abandoned cropland in Europe and North America, together with land in the former Soviet Union and ‘some areas of Africa’s savannas and South America’s Cerrado’ for this unavoidable increase in cultivated area.

One trend that can and should be challenged however is the global rush to biofuels. There is nothing inevitable about the increasing use of corn to produce ethanol or soybeans to manufacture biodiesel – these technologies offer no benefits to consumers, and have instead been driven by mistaken subsidies supposedly aimed at tackling climate change, but which instead most likely make it, and a whole host of other environmental problems, worse. Burning food crops for power is the worst use of scarce land imaginable, and has already led to a situation where there is a direct conflict between food and energy: a significant proportion of the food-price spike in 2008 (and a further spike in early 2011), which led to widespread hunger and bread riots in many poorer countries, was driven by crops being withdrawn from international markets to produce biofuels for transport.

Currently more than 40 per cent of the US corn crop goes into producing ethanol, which is mostly mixed with gasoline to fuel conventional cars. Even though this raises food prices and reduces the US surplus that can be sold on world markets, the ethanol industry is supported by a $6 billion subsidy scheme aimed at cutting greenhouse gas emissions. In Europe, biofuels are similarly supported with ‘environmental’ subsidies and fuel mandates. All these supports and promotion schemes for liquid biofuels should be scrapped, because of overwhelming scientific evidence that using land to produce energy crops delivers no climate benefits at all once agricultural emissions and land-use change are taken into account. This is obvious in the most egregious example of all: the clearing of tropical rainforest in Malaysia and Indonesia for oil-palm plantations, at least a third of which are used to produce feedstocks for biofuels (the rest goes into processed food, from chocolate to cooking oil, and cosmetics). Overall, 1.7 million hectares of Indonesian forest were converted to oil-palm plantation between 1990 and 2005, and the rate of destruction is accelerating. Scientists have calculated that the burning of peatland rainforest to free up land for plantations churns out more than 1,500 tonnes of carbon per hectare. Biofuels derived from cleared rainforest land should not just be discouraged – they should be outright banned.

Estimates of future land take for biofuels production range up to well over a billion hectares globally, more than double the 400 million hectares that remain if we are to respect the proposed planetary boundary. Once the need to produce more food is taken into consideration, it is clear that biofuels can only ever be a marginal contributor to world energy supplies. This conclusion has important implications. Most critically, liquid fossil fuels used in the world’s vehicle fleet of cars and trucks cannot simply be replaced with liquid biofuels. Instead, surface transport must be almost entirely converted to electricity. This is starting to happen already: all-electric cars or plug-in hybrids are beginning to hit the mass market, and will be particularly appropriate for urban or suburban drivers where the limited range of current battery technology is less of a concern. In the medium to long term, however, countrywide electrical-charging infrastructure will need to be built that allows electric cars to deliver all the range and refuelling convenience of petrol vehicles.

The only likely exception to the rule against biofuels in transportation is the urgent need to decarbonise air transport, where low-carbon alternatives to liquid hydrocarbon fuels remain a distant prospect. Whilst aviation has been demonised by environmentalists (myself included) in the past because of the climate-change impact of aircraft emissions, in terms of fuel efficiency per passenger kilometre the latest large aircraft like the Airbus A320 and the Boeing 787 now compare favourably with small family cars. The reason why per capita emissions from an intercontinental flight are counted in the many tonnes of CO2 is the enormous distances covered: no one drives from London to Sydney.

Therefore, with over 2 billion people using air travel every year already, and rapid uptake in developing countries like India and China, technical substitutes for high-carbon aviation must rapidly be found. If they can be sourced fairly sustainably, biofuels look promising, particularly ‘second-generation’ biofuels like algae that do not directly compete with food crops. (pdf) British Airways has led the way with its pioneering commitment in February 2010 to build a plant in the UK that will convert 500,000 tonnes of waste material into 16 million gallons of jet fuel annually. This may seem like a large amount, yet it represents only about 2 per cent of flights from London’s Heathrow Airport. This is the scale challenge of aviation, and demonstrates why biofuels may need to be almost exclusively reserved for air transport.

Where the electricity comes from to power the next generation of ground vehicles is also vitally important. Electric cars charging up using coal power will deliver little or no benefit to the climate. The best solution will be a dramatically upscaled combination of renewables and nuclear. Of course, the land-use implications of different energy options remain an issue – and here nuclear wins hands down. The reason why is basic: renewables work by harvesting diffuse energy like sunlight and wind over large areas, whilst nuclear fission delivers prodigious amounts of energy from tiny amounts of source material. Compared weight-by-weight, uranium 235 delivers a million times more energy than coal, which itself already represents chemical energy in a highly concentrated form. Just how much energy nuclear fission releases is described by Einstein’s famous equation E=MC2, where E is energy, M is mass and C the speed of light, about 300 million metres per second. Clearly even with a very small amount of fissionable material, multiplying it by the square of 300 million yields a very big number.

My argument is not in favour of nuclear and against renewables, however – both zero-carbon energy options, and various others, are essential to tackling climate change. Renewables can often be deployed in ways that minimise land-use and biodiversity impacts. Where building-mounted solar photovoltaics are cost-effective, no additional land – and therefore wildlife habitat – is used at all. Even the energy-hungry US could in theory supply all of its electricity use, according to one study, using solar PV on an area equivalent to 0.6 per cent of the country – just a fraction of the existing developed and urban space.

Sex and the city
Rural depopulation and urbanisation in developing countries are often decried by those who are concerned about the relentless expansion of megacities, which seem terribly unsustainable because of their noise, sprawling slums, congestion and pollution. But from the perspective of sustainable land use and habitat protection, the more that growing numbers of people can be persuaded to herd themselves into relatively small areas of urban land, the better for the environment.

In many parts of the world, if you want to marry the person you choose, be gay, be female and economically successful, or avoid daily backbreaking labour carrying water or fetching firewood, then you probably need to move to the city. In 1975 there were just three megacities of over 10 million people. Today there are 21. It sounds scary, but this unstoppable shift towards urbanisation actually ranks as one of the most environmentally beneficial trends of the last few decades. As the UN Population Fund wrote in a recent report: ‘Density is potentially useful. With world population at 6.7 billion people in 2007 and growing at over 75 million a year, demographic concentration gives sustainability a better chance. The protection of rural ecosystems ultimately requires that population be concentrated (pdf) in non-primary sector activities and densely populated areas.

City living is seldom lauded by environmentalists, but it may be our most environmentally friendly trait as a species, because urban dwelling is vastly more efficient than living in the countryside. Shops and other services are more concentrated in town and city neighbourhoods, and urban residents are much more likely to use public transport, share heating and housing, and have lower carbon footprints than their rural brethren. Given the scale of global population growth, the challenge still seems daunting: the world will need to accommodate 2 billion more urban dwellers (pdf) by 2030, a rate of expansion equivalent to building about 13 great cities (each with over 5 million inhabitants) per year, almost all in developing countries.

This gainsays conventional environmental wisdom in several ways. Clearly, the best strategy to curb future population growth is to speed up the ‘demographic transition’ in developing countries – and this transition towards women having fewer babies is inextricably linked both with increasing levels of prosperity and with urbanisation. Therefore rising rates of economic growth and the expansion of cities are good news for the environment because they will restrain the future growth in human population. Moreover, although the idea of getting close to the land in small-scale communities has a deep cultural resonance in some schools of environmentalist thought, in reality this is probably the worst thing that anyone can do.

All around the world, rural depopulation is leading to forest regrowth in abandoned areas – from the vast tracts of secondary broadleaf woodland in America’s New England states to tropical forests in Puerto Rico, the Dominican Republic and many other areas. In Costa Rica, abandoned cattle pasture is nurturing a flourishing young forest that in turn now supports a stable population of jaguars and other threatened fauna. A recent scientific paper looking at Latin America lists ‘similar patterns of ecosystem recovery following rural-urban migration’ in Patagonia, northwest Argentina, Ecuador, Mexico, Honduras and the montane deserts and Andean tundra ecosystems of Bolivia, Argentina and Peru. Even in rich countries, proposals for ‘rewilding’ – which I strongly support – only stand a chance of success in areas where rural populations have collapsed and formerly subsidised unproductive farms can be shut down to allow them to revert to nature.

This suggests that rural depopulation should not necessarily be opposed with ‘sustainable development’ schemes aimed at improving rural life to stop people migrating to cities. Equally, instead of encouraging low-tech traditional farming methods it may be preferable to focus on improving high-yield mechanised agriculture on the most fertile farmland to feed the new urban residents, whilst allowing mountainsides and other marginal lands to revert to forest. This is already happening by default in Latin America and elsewhere: in Vietnam, forest area has been increasing since the 1990s after small-scale, unproductive agriculture was made uncompetitive by more intensive, larger-scale farming in the more open market economy.

As always, one should not oversimplify. Cities themselves consume resources, including food, timber, water and energy, harvested over vastly wider areas than the land that they physically occupy, and this greater footprint needs to be considered in any overall assessment to get a true picture. When peasants move to the cities, their land might just as easily be turned over to large-scale cattle ranching or plantations as allowed to revert to forest. Studies have suggested that this is particularly the case in Amazonia, where most deforestation is carried out by ranchers, so population density is less clearly linked with the fate of the forest. Moreover, even after people move out, the recovery of forests cannot always be left to chance – it needs active management and ecologically friendly government policies. Whether secondary forest can help avoid large-scale species extinctions also depends on the extent to which animals and plants accustomed to old-growth forests can successfully recolonise new areas.

But the overall conclusion seems to me irrefutable. Urbanisation is good for sustainability, because it reduces population growth and concentrates the overall human impact on the land in a smaller area. Handled properly, migration away from rural areas and into cities offers a huge opportunity for ecosystem protection and restoration. Our best hope is therefore to encourage the trends towards rising prosperity and demographic transition in developing countries, in order to allow their forests and other important natural habitats to survive and regrow. Forget the ‘back to the land’ self-indulgence of some disgruntled people in rich countries. Billions of people want to move to urban areas to achieve increasing prosperity and improve their standard of living. Let us be glad of that. They are unwitting ‘Greens’, whose efforts at self-improvement should be celebrated.